Does anyone know what kind of power the 1LE axles are rated for? I haven't found any info on that.

426 through a 3.91 final drive and MM6 Tremec.

The SS M10 Tremec has a shorter first gear, so it will be interesting to see if a stripper SS could holeshot a 1LE. The MM6's close ratio gears though would probably even up or better the run after a few gear changes.

It's more than just a hp rating when considering axles. It's matching the torque at the axles to an axle with just the right twist properties. You don't want too much or too little, which is why you see different axles on each side of an OE application. They've tested out the optimum axle for each side. If you just through hugely oversized 1400 hp axles in both sides, in an otherwise stock driveline, expect to break diffs, driveshafts... The axles will probably hold just fine, but now your moving the twist/wrap up to other parts that weren't designed for it. Getting the right axle twist is also key to avoiding wheelhop.

Another thing to look into is any mods done to the 1LE driveshaft, such as bigger bolts. Probably a good reason those got upgraded also.